Acid Recovery

ABSTRACT

Nickel is recovered from a nickeliferous ore, where the ore includes impurities in the form of metals other than nickel and cobalt. The ore is subjected to atmospheric leaching using sulphuric acid to dissolve nickel and cobalt from the ore and produce a leach liquor. The sulphuric acid is regenerated by rapidly heating the leach liquor to a temperature in excess of 120° C. to produce a slurry including sulphuric acid and precipitated oxides of impurities present in the leach liquor, and separating the precipitated oxides from the slurry to produce a clarified acid rich liquor stream for recycle to leaching.

FIELD OF THE INVENTION

The present invention relates to a process for regeneration of sulphuricacid from leach liquors generated by atmospheric leaching ofnickeliferous ore, wherein the ore includes impurities in the form ofmetals other than nickel and cobalt, and wherein the ore is subjected toatmospheric leaching using sulphuric acid to dissolve nickel and cobaltfrom the ore and produce a leach liquor.

BACKGROUND TO THE INVENTION

Nickel ores can be classified into two major types according to theircomposition, namely, sulfide and laterite (the latter being also knownas “oxidized”). Until recently, the majority of total nickel productioncame from processing of sulfide ores. In more recent times, lateriteores have started to be treated on a commercial scale as a consequenceof higher-grade nickel sulphide deposits being mined out, making themore complex laterite ores economically viable to process. The nickeland cobalt metals present in the laterite ore can be recovered usingleaching to take nickel and cobalt into solution for subsequentrecovery.

Traditionally, the laterite ores have been treated via an expensive,energy intensive and highly corrosive high pressure sulphuric acidleaching, smelting or the Caron processes. More recently, leachingcircuits conducted at atmospheric pressure have been proposed. Forexample, heap leaching is an operation that involves low investment andcircuital costs compared with high pressure leaching, is very wellknown, and is widely applied mainly for copper, uranium, and gold ores.Heap leaching is conducted at atmospheric pressure and ambienttemperature. Dilute acid solution is irrigated over the top of the heapand leaching proceeds for an extended period of time as the nickel andcobalt are dissolve into a pregnant leach liquor which is collected fromthe base of the heap. Similarly, it is known that laterite ores can beleached in slurry form at atmospheric pressure and elevated temperature,typically above 70° C. In either case, the resultant pregnant leachliquors can be treated to recover the dissolved nickel and cobaltvalues.

In all cases of such atmospheric acid leaching, excess acid is consumedin dissolving iron, aluminium, chrome, copper, and manganese which arealso present in the laterite ores in various concentrations.

Prior art processes have been developed to separate the iron andaluminum impurities from pregnant leach liquors by causing theimpurities to precipitate out of the pregnant liquor by addingneutralizing reagents to alter the pH of the pregnant leach liquor (see,for example U.S. Pat. No. 3,720,749; U.S. Pat. No. 3,991,159; U.S. Pat.No. 4,097,575 and U.S. Pat. No. 4,547,348). The most commonly usedneutralizing agent are magnesia (MgO) and calcium carbonate richreagents such as limestone or coral mud, which is typically added to thepregnant liquor in a sufficient quantity to adjust the pH of the liquorwithin the narrow pH range of about 3.5 to 4.5 at a temperature of about50 to 100° C. to at atmospheric pressure. Following the precipitationand separation of the aforementioned impurities, the pH of the pregnantor leach liquor is then raised to at least 7 by the further addition ofmagnesia to cause the nickel and cobalt present in the pregnant leachliquor to precipitate out, allowing the nickel and cobalt-richprecipitates to be separated from the liquor.

It is also known to attempt to minimize consumption of both sulphuricacid and/or the neutralizing agent by separating the nickel bearing oreinto high and low magnesium fractions prior to leaching, and subjectiononly the low magnesium fraction to leaching, whilst using the highmagnesium fraction or raw ore itself as some or all of the neutralizingagent (see, for example, U.S. Pat. No. 4,097,575; U.S. Pat. No.3,804,613; U.S. Pat. No. 3,991,159; and, U.S. Pat. No. 4,044,096).

At the conditions under which these prior art processes are operated,aluminum and iron values precipitate out primarily as gelatinoushydroxides and basic sulfates. These precipitates are not very dense andtend to be voluminous, with the result that particular care must betaken to separate these precipitates from the pregnant leach liquorusing solid-liquid separation techniques well known in thehydrometallurgical art. These prior art methods are expensive in termsof the costs associated with the consumption of the neutralisingreagents. More importantly, such prior art processes only exacerbate theproblems associated with excessive consumption of the acid by theimpurities present in the ore.

With atmospheric leaching methods becoming more attractive forprocessing nickel laterite ores, there remains a need for a processwhich reduces fresh acid consumption to overcome or at least amelioratethe above-identified problems with the prior art.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod of recovering nickel from a nickeliferous ore, wherein the oreincludes impurities in the form of metals other than nickel and cobalt,and wherein the ore is subjected to atmospheric leaching using sulphuricacid to dissolve nickel and cobalt from the ore and produce a leachliquor, the sulphuric acid is regenerated using a method comprising:

-   -   a) rapidly heating the leach liquor to a temperature in excess        of 120° C. to produce a slurry comprising sulphuric acid and        precipitated oxides of impurities present in the leach liquor;        and,    -   b) separating the precipitated oxides from the slurry of step a)        to produce a clarified acid rich liquor stream for recycle to        leaching.

Step a) may be conducted at a temperature in the range of 120 to 220° C.or in the range of 180 to 220° C. to initiate the precipitation ofoxides of impurities by hydrolysis.

Advantageously, leach liquors with high iron contents can be treated inthat the leach liquor of step a) may have at least 10 g/L or at least 25g/L of dissolved iron.

Preferably, the precipitated oxides are hematite or alunite.

In one form, the leach liquor of step a) has an initial free acidconcentration of at least 10 g/L free acid.

To achieve rapid heating, step a) may be conducted using an autoclavewhich operates in combination with one or more pre-heating stagesarranged to receive steam that is generated from cooling or “flashing”when the slurry discharged from the autoclave is depressurized. Theautoclave may include one or more steam injection points arrangedreceived fresh steam to control the temperature within the autoclave inuse.

In one form of the present invention, the leach liquor of step a) is apregnant leach liquor. The pregnant leach liquor may be subjected to ametals recovery process to recover nickel and cobalt from the pregnantleach liquor and form a barren leach liquor. The metals recovery processmay be selected from the group consisting of sulphide precipitation withhydrogen sulphide gas, solvent extraction, ion exchange, or acombination thereof. When the metals recovery process is sulphideprecipitation, sulphide precipitation may be conducted on a pregnantleach liquor having a level of acidity of less than 10 g/L free acid ata temperature in the range of 80 to 120° C. If required, a neutralizingagent may be added to a bypass stream of the clarified acid-rich liquorstream of step b) to achieve a target free acid content in a neutralizedpregnant liquor stream within the range of 1-10 g/L of free acid priorto conducting sulphide precipitation. In this form of the presentinvention, the neutralized pregnant liquor stream may then be directedto a metals recovery process. In one form, the neutralizing agent iscalcium carbonate. The portion of clarified acid-rich leach liquor ofstep b) which forms the bypass stream may be determined by ensuring thatthe target amount of nickel recovered using the metals recovery processis equal to the amount of nickel being leached per pass duringatmospheric leaching.

Preferably, acid recovery is conducted after metals recovery and theleach liquor of step a) is a barren leach liquor.

In one form of the present invention, atmospheric leaching may beconducted using a heap leaching process.

According to a second aspect of the present invention there is provideda method of regenerating sulphuric acid substantially as hereindescribed with reference to and as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a more detailed understanding of the nature ofthe invention several embodiments of the present invention will now bedescribed in detail, by way of example only, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic flowchart illustrating a first embodiment of theacid recovery process of the present invention in which metals recoveryoccurs prior to acid recovery; and,

FIG. 2 is a schematic flowchart illustrating a second embodiment of theacid recovery process of the present invention in which metals recoveryoccurs after acid recovery.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Particular embodiments of the method of regenerating sulphuric acid forre-use in heap leaching to recover nickel and cobalt from anickeliferous ore. The present invention is equally applicable to therecovery of acid from leach liquors produced using atmospheric leachingoperations other than heap leaching. The terminology used herein is forthe purpose of describing particular embodiments only, and is notintended to limit the scope of the present invention. Unless definedotherwise, all technical and scientific terms used herein have the samemeanings as commonly understood by one of ordinary skill in the art towhich this invention belongs. In the drawings, it is to be understoodthat like reference numbers refer to like parts. The abbreviation “° C.”as used throughout this specification refers to “degrees Celsius”.

The term “leaching” as used throughout this specification refers to theselective dissolution of a target metal contained in an insoluble solidphase using a solvent or “leaching solution”. For leaching to occur, anore to be leached is brought into contact with the solvent, taking thetarget metal value(s) into solution. Any metals other than the targetmetal which dissolve in the leach solution are referred to throughoutthis specification as “impurities”. By way of example, during leachingof laterite ores, the target metals are nickel and (optionally) cobalt,whilst iron, aluminium, chromium, copper and manganese are considered tobe impurities.

Leaching results in the formation of “a pregnant leach liquor”, which isa term that is used throughout this specification to refer to a leachliquor which is rich in target metal values. After leaching has beencompleted, target metals are extracted from the pregnant leach liquorusing a “metals recovery process”. Downstream of the metal recoveryprocess, after the target metals have been removed, the leach liquor isreferred to throughout this specification as “a barren leach liquor”.

“Atmospheric Leaching” is a process which is conducted at ambientpressure (in contrast to a high pressure leaching process which requiresspecialist equipment such as pressure vessels or autoclaves). “HeapLeaching” is a process in which the ore is piled up to form a “heap” andthe target metals are taken into solution by percolation of the leachingsolution through the heap.

One embodiment of a process (10) for regeneration of acid for use inleaching nickel-containing laterite ores is now described with referenceto FIG. 1, in which a pregnant leach liquor (12) for treatment isobtained using a leaching circuit (14), in this example, a heap leachingcircuit. Agglomerated pellets of nickel-containing laterite ore aredirected from a stacker to form one or more heaps, each heap having abase and a top. Each heap can be a static or dynamic (“on-off”) type ofheap. If desired, a counter-current series of heaps can be usedincluding a lead heap and a lag heap, with fresh leaching solution beingintroduced at the top of the lag heap. Alternatively, the pellets can bestacked into vessels or vats, which should be considered to consist of aheap having a limited, fixed wall size. A distribution means, forexample a spraying apparatus, is used to apply a leaching solution orsolvent at the top of the heap. The leaching solution is then allowed topercolate down through the heap under the influence of gravity towardsthe base of the heap.

A suitable leaching solution is any reagent capable of selectivelydissolving a target metal from an ore whilst remaining chemically inertto gangue minerals. The leaching solution preferably contains sulphuricacid. When sulphuric acid is used, the leach solution typically containsbetween 0.1 and 20% sulphuric acid. Optimum recovery of nickel duringheap leaching is achieved when about 400 to about 1000 kilograms ofsulphuric acid per tonne of dry ore is consumed, without acidregeneration. The acid solution may be applied by spraying it onto alayer of inert bedrock, which acts as liquid distributor on the heap,although any method to adequately and uniformly disperse the acid ontothe heap can be used. The acid solution is applied at an irrigation rateor flux in the range of 5 to 30 litres per hour per meter squared,preferably 10 to 25 litres per hour per meter squared.

Pregnant leach liquor (12) is collected using a collection meansarranged at the base of each heap. The pregnant leach liquor is eitherrecycled for a second pass through the same heap or another heap in theseries, or directed to a metals recovery circuit (16) to recover one orboth of the nickel values and the cobalt values present in the pregnantleach liquor. After leaching has been completed, the heap may be rinsedwith water (either fresh or salt water), concentrated or dilute acid ora combination of both of these, with an effluent stream being collectedfrom the base of the heap after rinsing. If desired, this effluentstream can be directed to the metals recovery circuit (16) in additionto the pregnant leach liquor to improve overall recovery of nickeland/or cobalt values.

Any suitable metals recovery process known in the art, such asprecipitation, solvent extraction or ion exchange, can be used to in themetals recovery circuit (16) to separately recover dissolved targetmetal values such as nickel and cobalt from the pregnant leach liquor ata satisfactory purity. By way of example, the nickel and cobalt presentin the pregnant leach liquor (12) can be removed through the addition ofa source of sulphide ions, for example by introducing a hydrogensulphide gas stream or through the addition of sodium hydrosulphide(NaHS) in slurry form. Other metal recovery methods can equally be used,for example, hydroxide precipitation, solvent extraction or ionexchange. The addition of the sulphide ions under certain conditionscauses the nickel and cobalt metal values present in the pregnant leachliquor (12) to precipitate. Precipitation occurs at temperatures in therange of 80 to 120° C. provided that, prior to the introduction of thesource of sulphide ions, the acidity of the pregnant leach liquor (12)is maintained at a level of acidity of less than 10 g/L free acid.

Precipitation of the nickel or cobalt occurs according to the followingsimplified general equation, where “M” is a divalent metal:

MSO₄(aq)+H₂S→MS(s)+H₂SO₄   (1)

It is clear from equation (1) above, that the acidity of the leachliquor increases as this precipitation reaction progresses. For apregnant leach liquor with an initial acidity of 1 g/L, the finalacidity after nickel and cobalt precipitation can be as high as 10 g/L.The precipitated solids of nickel and cobalt are removed from the leachliquor after precipitation using any suitable solid/liquid separationequipment (which forms part of the metals recovery circuit (16)) toproduce a clarified barren liquor stream (18).

In the embodiment illustrated in FIG. 1, the clarified barren liquorstream (18) from the metals recovery circuit (16) is directed to an acidrecovery circuit (20). The clarified barren leach liquor stream (18)contains dissolved impurities selected from the group consisting ofiron, aluminum, chrome and manganese, in various concentrationsdepending on the starting composition of the laterite ore that wassubjected to leaching. The most significant impurities from the point ofview of acid consumption and regeneration in the leaching circuit (14)are iron and aluminium. Using the process of the present invention, thesulphuric acid which was consumed when the impurities were taken intosolution in the leaching circuit (14) is regenerated from the leachliquor using the acid recovery circuit (20) by causing these impuritiesto precipitate as oxides. Acid recovery is achieved by rapidly heatingthe leach liquor to a temperature in the range of 120 to 220° C.,preferably in the range of 180 to 220° C. to initiate the rapidprecipitation of iron and aluminum from the leach liquor by hydrolysis.Under these conditions, the precipitates formed are predominately oxidesof the impurities. It is important to note that the target metal valuesof nickel and cobalt do not precipitate from the leach liquor underthese conditions. However, some loss of nickel and cobalt can occur byentrainment during subsequent solid/liquid separation operations, whenthe metals recovery circuit (16) is located downstream of the acidrecovery circuit (20) as described below in relation to an alternativeembodiment of the present invention which is illustrated in FIG. 2.

By way of example, sulphuric acid is regenerated when iron and aluminiumimpurities present in the leach liquor precipitate as oxides accordingthe following simplified reactions:

Fe₂(SO₄)₃(aq)+3H₂O→Fe₂O₃(s)+3H₂SO₄   (2)

3Al₂(SO₄)₃(aq)+13H₂O→3Al₂O₃.4SO₃.8H₂O (s)+5H₂SO₄   (3)

It is clear from equations (2) and (3) above, that when the oxides arecaused to form, sulphuric acid is recovered from the leach liquor. Byway of example, when the acid level of the leach liquor being treatedhas an initial free acid concentration of approx 10 g/L free acid, thefinal free acid concentration after causing rapid precipitation of theimpurities from the leach liquor increases to over 50 g/L free acid. Thepractical outcome of this is that up to half of the acid used to conductleaching can be recovered and made available for return to the leachingcircuit (14).

The precipitated oxides are removed using a solid/liquid separationcircuit (22) to produce a clarified acid-rich liquor stream (40) whichis available for re-cycle to the leaching circuit (14). The oxidesremoved from the liquor stream using the solid/liquid separation circuit(22) are disposed of. Solid/liquid separation in the solid/liquidseparation circuit (22) can be achieved using any suitable process knownin the hydrometallurgical art, for example; thickening, filtering,pressure filtering, centrifugal separation, gravity separation,counter-current decantation, or cyclones. Unlike the hydroxide and basicsulphate precipitates of the prior art, the oxides which precipitate inthe acid recovery circuit (20) of the present invention form dense solidparticles which settle well and are readily filterable.

Rapid heating in the acid recovery circuit (20) can be achieved in anumber of ways. In the embodiments illustrated in FIGS. 1 and 2, rapidheating is achieved using an autoclave (26) which operates incombination with one or more pre-heating stages (24) arranged to receivesteam that is generated from cooling or “flashing” when the slurrydischarged from the autoclave (26) is depressurized. By way of example,the autoclave (26) can be a brick lined or titanium lined pressurevessel with multiple compartments, each compartment being fitted with anagitator (28) to improve reaction kinetics. The autoclave (26) has oneor more steam injection points (30) arranged to control the temperaturewithin the autoclave (26) in use within the predetermined temperaturerange referred to above to encourage acid regeneration through theprecipitation of oxides such as hematite (Fe₂O₃) and alunite(Al₂O₃.4SO₃.8H₂O) in accordance with equations (2) and (3) respectively.

After precipitation of the oxides from the liquor stream, a slurrystream (32) is discharged from the autoclave (26) into one or morestages of pressure let-down using a series of cooling vessels or flashtanks (34). Within each of the flash tanks (34), the hot pressurisedslurry stream (32) is caused to pass through a choke valve (not shown)causing rapid acceleration which leads to a drop in pressure. This rapiddrop in pressure causes some of the water present in the slurry to flashto steam. The steam that forms within each of the flash vessels (34),apart from the final flash vessel (35), is ducted to the pre-heatingstages (24) for heat recovery. Two such flash tank (34)/pre-heatingstages (24) are illustrated in each of the embodiments of FIG. 1 andFIG. 2, but this number can vary. The specific number of stages willdepend on the cost of energy versus the capital cost of the equipment.The final flash vessel (35) is operated at atmospheric pressure and thesteam from this final flash vessel (35) is vented to atmosphere as it isunsuitable for heat recovery. A stream of fresh steam (37) is usedmaintain temperature control, with a portion of the fresh stream beingdirected to the final pre-heating stage (25) and a portion of the freshsteam (37) being directed to the steam injection points (30).

The cooled slurry stream (36) downstream of the flash tanks (34), at anominal temperature in the order of 100° C., is then discharged to thesolid/liquid separation circuit (22). The precipitated oxides of theimpurities (38) removed from the cooled slurry stream (36) using thesolid/liquid separation circuit (22) are discarded. The clarifiedacid-rich liquor stream (40) which is recovered from the cooled slurrystream (36) using the solid/liquid separation circuit (22) has a highfree acid content which makes it suitable for recycle to the leachingcircuit (14).

The acid recovery circuit (20) can be located either upstream ordownstream of the metals recovery circuit (16). In the flowchart of FIG.1, the acid recovery circuit (20) is located downstream of the metalsrecovery circuit (16) with the result that the leach liquor beingtreated in the acid recovery circuit (20) is a barren leach liquor. Inthe flowchart of FIG. 2, the acid recovery circuit (20) is locatedupstream of the metals recovery circuit (16) with the result that theleach liquor being treated in the acid recovery circuit (20) is apregnant leach liquor. For best results, the metals recovery process(16) is conducted prior to the acid regeneration process (20). One ofthe reasons for this is that conducting the metals recovery process (16)first minimizes the loss of nickel and cobalt values which can occur dueto entrainment when the precipitated oxides are removed from the leachliquor in the solid-liquid separation circuit (22).

In the embodiment illustrated in FIG. 2 for which like referencenumerals refer to like parts, the acid recovery circuit (20) is arrangedto receive the pregnant leach liquor stream (12) produced by theleaching circuit (14). The clarified acid-rich liquor stream (40)produced from the solid/liquid separation circuit (22) has a high freeacid content of at least 50 grams per litre, which makes the clarifiedacid-rich liquor stream (40) unsuitable for sulphide precipitationwithout first taking steps to pre-neutralize the liquor stream (40)prior to the addition of the source of sulphide ions. The reason forthis is that the sulphide precipitation method which proceeds accordingto equation (1) above is only effective in causing the precipitation ofnickel and cobalt from a pregnant leach liquor which has a relativelylow free acid content in the range of approximately 1-10 g/L free acid.

With reference to FIG. 2, a bypass stream (50) of the acid-rich liquorstream (40) downstream from the solid-liquid separation circuit (22) isdirected to a pre-neutralization circuit (52). In the pre-neutralizationcircuit (52), a neutralizing agent, for example, limestone is added toachieve a target free acid content in a neutralized pregnant liquorstream (54) within the range of 1-10 g/L of free acid. Addition oflimestone in this manner causes the precipitation of gypsum from theneutralized pregnant liquor stream (54) according to the followingsimplified equations:

CaCO₃+H₂SO₄→CaSO₄(aq)+CO₂+H₂O   (4)

When the neutralized pregnant liquor stream is saturated with aqueouscalcium sulphate, the calcium sulphate precipitates as gypsum inaccordance with the following equation:

CaCO₃+H₂O→CaSO₄.H₂O   (5)

Following separation of the gypsum solids, a neutralized pregnant liquorstream (54) is directed to the metals recovery circuit (16). A suitablemetals recovery process, such as sulphide precipitation, is then used inthe metals recovery circuit (16) to recover nickel and cobalt valuesfrom the neutralized pregnant liquor stream (54). The remaining portionof the clarified acid-rich leach liquor (40) from the solid-liquidseparation circuit (22) has a high free acid content, making it suitablefor recycle to the leaching circuit (14).

In the embodiment illustrated in FIG. 2, the clarified acid-rich leachliquor (40) is low in impurities such as iron and aluminium, which havebeen removed as precipitated oxides in the acid recovery circuit (20),but still contains significant levels of nickel and cobalt in solutionbecause it has not been subjected to a metals recovery process. Theportion of clarified acid-rich leach liquor (40) which forms the bypassstream (50) is determined by ensuring that the target amount of nickelrecovered during the metals recovery circuit (16) is equal to the amountof nickel being leached from the heap per pass. For example if theclarified leach liquor (50) from the solid-liquid separation circuit(22) contained 3 g/L of nickel, and 2 g/L of nickel was leached from theheap per pass during leaching circuits (14), giving a total resultantgrade of 5 g/L, the portion of the clarified acid-rich leach liquor (40)to be subjected to pre-neutralisation and metals recovery circuits (52)and (16), respectively, would be around 40% of the clarified acid-richleach liquor (40).

The present invention is particularly suited to the treatment of leachliquors containing at least 10 g/L of dissolved iron and preferably atleast 25 g/L of dissolved iron.

The advantages of the various aspects and embodiments of the presentinvention are further described and illustrated by the followingexamples and experimental test results. These examples and experimentaltest results are illustrative of a variety of possible implementationsand are not to be construed as limiting the invention in any way. Thepresent invention is also not limited by the particular number nor typeof equipment described in the following examples.

EXAMPLES

A heap leach column test using sulphuric acid and laterite ore wasconducted to produce a leach liquor containing target metal values inthe form of nickel and cobalt in solution. The leach liquor alsocontains dissolved iron, aluminium, chrome, magnesium and manganeseimpurities.

Approximately 3000 grams of the leach liquor was placed in a titaniumParr autoclave, which was then sealed and heated to 220° C. Afterachieving this temperature, samples of the solution were taken after 15minutes. Thereafter, the autoclave was allowed to cool. After coolingthe pulp was weighed and filtered to recover precipitated solids. Theinitial leach liquor from heap leaching, the solids recovered duringfiltering and the leach liquor after precipitation were analysed todetermine the free acid levels and composition of each, with the resultsbeing presented below in Table 1.

TABLE 1 Filtered Product Feed Solution Solution % Precipitated Free Acid9.8 80.5 — (g/l) Nickel 4379 4259 0.33 (mg/l) Cobalt 327 297 0.70 (mg/l)Iron 33220 4084 81.7 (mg/l) Aluminium 7791 893 84.4 (mg/l) Chrome 732287 53.5 (mg/l) Magnesium 21970 22830 0.00 (mg/l) Manganese 1522 14360.07 (mg/l)

Now that several embodiments of the invention have been described indetail, it will be apparent to persons skilled in the relevant art thatnumerous variations and modifications can be made without departing fromthe basic inventive concepts. For example, a portion of the steamgenerated by the flash tanks (34) can be directed to one or all of steaminjection points (30) of the autoclave (26) if desired. All suchmodifications and variations are considered to be within the scope ofthe present invention, the nature of which is to be determined from theforegoing description and the appended claims.

All of the patents cited in this specification, are herein incorporatedby reference. It will be clearly understood that, although a number ofprior art publications are referred to herein, this reference does notconstitute an admission that any of these documents forms part of thecommon general knowledge in the art, in Australia or in any othercountry. In the summary of the invention, the description and claimswhich follow, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

1. In a method of recovering nickel from a nickeliferous ore, whereinthe ore includes impurities in the form of metals other than nickel andcobalt, and wherein the ore is subjected to atmospheric leaching usingsulphuric acid to dissolve nickel and cobalt from the ore and produce aleach liquor, the sulphuric acid is regenerated using a methodcomprising: a) rapidly heating the leach liquor to a temperature inexcess of 120° C. to produce a slurry comprising sulphuric acid andprecipitated oxides of impurities present in the leach liquor; and, b)separating the precipitated oxides from the slurry of step a) to producea clarified acid rich liquor stream for recycle to leaching.
 2. Themethod of regenerating sulphuric acid of claim 1, wherein step a) isconducted at a temperature in the range of 120° C. to 220° C. toinitiate the precipitation of oxides of impurities by hydrolysis.
 3. Themethod of regenerating sulphuric acid of claim 1, wherein step a) isconducted at a temperature in the range of 180° C. to 220° C. toinitiate the precipitation of oxides of impurities by hydrolysis.
 4. Themethod of regenerating sulphuric acid of claim 1, wherein the leachliquor of step a) has at least 10 g/L of dissolved iron.
 5. The methodof regenerating sulphuric acid of claim 1, wherein the leach liquor ofstep a) has at least 25 g/L of dissolved iron.
 6. The method ofregenerating sulphuric acid of claim 1, wherein the precipitated oxidesare hematite or alunite.
 7. The method of regenerating sulphuric acid ofclaim 1, wherein the leach liquor of step a) has an initial free acidconcentration of at least 10 g/L free acid.
 8. The method ofregenerating sulphuric acid of claim 1, wherein step a) is conductedusing an autoclave which operates in combination with one or morepre-heating stages arranged to receive steam that is generated fromcooling or “flashing” when the slurry discharged from the autoclave isdepressurized.
 9. The method of regenerating sulphuric acid of claim 8,wherein the autoclave includes one or more steam injection pointsarranged received fresh steam to control the temperature within theautoclave in use.
 10. The method of regenerating sulphuric acid of claim1, wherein the leach liquor of step a) is a pregnant leach liquor. 11.The method of regenerating sulphuric acid of claim 10, wherein thepregnant leach liquor is subjected to a metals recovery process torecover nickel and cobalt from the pregnant leach liquor and form abarren leach liquor.
 12. The method of regenerating sulphuric acid ofclaim 11, wherein the metals recovery process is selected from the groupconsisting of sulphide precipitation with hydrogen sulphide gas, solventextraction, ion exchange, and combinations thereof.
 13. The method ofregenerating sulphuric acid of claim 11, wherein the metals recoveryprocess is sulphide precipitation conducted on a pregnant leach liquorhaving a level of acidity of less than 10 g/L free acid at a temperaturein the range of 80° C. to 120° C.
 14. The method of regeneratingsulphuric acid of claim 10, wherein a neutralizing agent is added to abypass stream of the clarified acid-rich liquor stream of step b) toachieve a target free acid content in a neutralized pregnant liquorstream within the range of 1 g/L to 10 g/L of free acid.
 15. The methodof regenerating sulphuric acid of claim 14, wherein the neutralizedpregnant liquor stream is directed to a metals recovery process.
 16. Themethod of regenerating sulphuric acid of claim 14, wherein theneutralizing agent is calcium carbonate.
 17. The method of regeneratingsulphuric acid of claim 14, wherein the portion of clarified acid-richleach liquor of step b) which forms the bypass stream is determined byensuring that the target amount of nickel recovered using the metalsrecovery process is equal to the amount of nickel being leached per passduring atmospheric leaching.
 18. The method of regenerating sulphuricacid of claim 1, wherein the leach liquor of step a) is a barren leachliquor.
 19. The method of regenerating sulphuric acid of claim 1,wherein atmospheric leaching is conducted using a heap leaching process.